Wet cell rechargeable batteries or secondary batteries such as lead-acid, silver-cadmium, nickel-cadmium, and silver-zinc deteriorate with age and usage because the plates shed active material and are chemically and mechanically altered during the charge-discharge cycles of normal operation. Such gradual deterioration ultimately results in a device that no longer can retain a useful charge of sufficient capacity to carry its normal load. The inconvenience of sudden and unanticipated battery failure is well known to operators of automobiles, aircraft, industrial traction trucks, and military vehicles. A battery-condition monitor that indicates the state of deterioration and which is capable of signalling the approaching end of useful life provides a useful expedient to the users of equipment dependent upon the proper operation of associated rechargeable batteries.
Heretofore, numerous approaches have been taken to provide a device capable of indicating the state of deterioration of such batteries. Approaches have been taken wherein a test for the conductivity of the electrodes of the battery is run since such conductivity changes with cycling and charge. Such an approach is set forth in U.S. Pat. No. 2,988,590. Yet other devices have been proposed such as in U.S. Pat No. 3,065,827 wherein voltage sensitive devices such as relays are provided to actuate charge indication means, such as lights, if the battery voltage is above or below a particular level. A similar approach, utilizing a warning light, is presented in the U.S. Pat. No. 3,118,137 wherein an electronic circuit is connected across the terminals of a battery to continually monitor the terminal voltage thereof and determine the state of the charge remaining in the battery as a function of the terminal voltage. This prior art structure requires a constant monitoring of the battery potential and consequent battery drain along one of two discharge paths.
Many other techniques and devices for the testing of battery conditions have been proposed wherein the specific gravity and/or chemical composition of the battery electrolyte is tested. Such tests are generally complex, inaccurate, and conducive only to a manual rather than automatic mode of operation. Other techniques of electronic analysis of battery condition are proposed in U.S. Pat. Nos. 3484681; 3500167; 3503062; 3529230; 3550105; and 3832629. While these references are of interest for purposes of general prior art concepts, the shortcomings thereof will be readily apparent in view of the embodiments of the instant invention presented herein.
It is an object of the instant invention to present a battery condition monitor for attachment to a vehicle battery and operative to test the condition of such battery at the instant of loading the same. This test is truly a measure of battery capability to service the existing load, at the instant of loading.
Still another object of the invention is to present a battery condition monitor wherein the transient voltage of the battery, under initial load, is tested and the general condition of the battery is determined thereby.
Yet another object of the invention is to verify subsystem integrity by testing loading situations that may be temporarily serviced by the battery but are detrimental to its service life.
Yet another object of the invention is to present a battery condition monitor wherein a basic testing technique on a lead cell battery is adaptable for any of numerous implementations.
Still a further object of the invention is to provide a battery condition monitor which is inexpensive to construct, reliable in operation, highly accurate in testing, has a low power standby posture, and is conducive to implementation in any of a multitude of systems incorporating rechargeable batteries.
These objects and other objects which will become apparent as the detailed description proceeds are achieved by: A battery condition monitoring device for connection across the terminals of a rechargeable battery, comprising: a function generator connected to the battery and producing an output signal as a function of the terminal voltage of the battery; a timing circuit connected between the battery and function generator for determining the time period for the output signal to reach a particular level; comparison means connected to said timing circuit for comparing the time period with preselected time references, the comparisons determining the condition of said battery; and display means connected to said comparison means for indicating the condition of said battery.